Laser beam has been used in various applications (e.g. measurement and communication). Especially, a semiconductor laser with a light source of a laser diode and the like is used in a field where stability in strength and wavelength of the laser beam is desired.
Such a semiconductor laser is provided by a laser source device including a laser source element for emitting a laser beam and an optical element disposed downstream of the laser source element.
The optical element used herein is exemplified by a collimating lens, an optical filter such as an etalon plate, polarization plate and various wave plates, a prism, a protection transparent plate and a combination thereof.
In the laser source device, the laser beam from the laser source element is sometimes reflected by the downstream optical element to return back to the laser source element (a return light).
Especially, when the surface of the optical element is flat, the strength of the return light increases in the same direction and, when there is a collimating lens, the return light passing through the collimating lens may sometimes converges to the laser source element.
When the return light is incident on the laser source element, the oscillation in the laser source element becomes unstable to disadvantageously change the strength and wavelength of the laser beam to be emitted.
In order to restrain the return light from being incident on the laser source element, an antireflection film on the optical element or an optical isolator has been provided.
Among the above, when an antireflection film is provided to an optical element, the transmittance of the optical element can be enhanced to reduce the return light.
However, a large cost is required for providing a high-transmittance antireflection film and the transmittance-enhancement effect of the antireflection film is anyhow limited. In addition, the antireflection film cannot be formed on some of the optical elements.
On the other hand, the optical isolator is expensive and entails size enlargement and design complication of the laser source device.
In order to more easily restrain the return light, it has been proposed to incline the optical element with respect to a main optical axis of the laser beam (see, for instance, Patent Literature 1: JP-A-2014-11442).
It should be noted that the laser beam emitted from the laser source element has an astigmatic difference (i.e. a difference between a focus in a direction along an active layer and a focus in a direction intersecting the active layer), so that the beam shape (i.e. cross sectional shape of the beam) changes along the optical axis.
As shown in FIG. 6, a laser source device 90 includes a laser source element 91 that emits a laser beam 99 and a case 92 that houses the laser source element 91.
The laser source element 91 has an active layer disposed along Z and X directions in the figure, so that the laser beam 99 is emitted in the Z direction. The beam shape of the laser beam 99 is defined by a cross section along the X and Y directions.
A photodetector 93 for output control is disposed on the back side of the laser source element 91.
The beam shape of the laser beam 99 emitted from the laser source element 91 defines an ellipse having an X-direction length NX longer than a Y-direction length NY and a long axis along the X direction in a region near the laser source element 91 (near-field).
On the other hand, in a region far from the laser source element 91 relative to the near-field (far-field), the beam shape of the laser beam defines an ellipse having the Y-direction length FY longer than the X-direction length FX and the long axis along the Y direction (i.e. a direction intersecting the active layer)
The above-described change in the beam shape due to the astigmatic difference (i.e. the change in the long-axis direction of ellipse) is not considered in the above-described Patent Literature 1, and the return light cannot be sufficiently kept from being incident on the laser source element according to the solution of Patent Literature 1.
Specifically, the optical element may be tilted to, for instance, shift the direction of the return light toward the laser source element in the Y direction so as not for the return light to be incident on the laser source element.
However, the return light reflected by the optical element in the far-field forms an ellipseal spot having a long axis in the Y direction near the laser source element.
When the beam shape of the return light is an ellipse having a long axis in the direction, it is possible that a part of the return light is incident on the laser source element even when the return light is shifted in the Y direction, thus failing to emit a stable laser beam.
When the laser beam forms the ellipseal spot as described above, it is possible to prevent the return light from being incident on the laser source element by further increasing the installation angle of the optical element.
However, a further increase in the installation angle of the optical element entails an increase in the size of the device and consequent reduction in the rigidity and temperature-stability of the device.
Further, the installation angle cannot be greatly changed in some optical elements. For instance, when an etalon is used as the optical element, the transmissive wavelength and transmissive light volume change according to the installation angle, so that the installation angle cannot be greatly changed.